Optical fluoride crystals are useful materials because of their low-wavelength absorption edges. Optical fluoride crystals such as CaF2, BaF2, SrF2, LiF, MgF2, and NaF are useful in applications that require high transmission in the vacuum ultraviolet (VUV) region, i.e., at wavelengths below 200 nm. Optical fluoride crystals are particularly useful as below 200 nm transmitting optical element preforms for formation into optical elements with below 200 nm transmission, such as VUV microlithography optical elements (lens, prisims). Optical fluoride crystals are commonly directionally solidified by the Bridgman-Stockbarger technique. Other techniques for growing optical fluoride crystals include the Gradient Freeze technique and the Traveling Heater technique.
The Bridgman-Stockbarger technique is illustrated in FIGS. 1A and 1B. In FIG. 1A, a crucible C containing a fluoride raw material F is disposed inside a hot zone HZ of a vertical furnace 1. Heaters 2 are provided to heat the hot zone HZ to a temperature sufficient to melt the fluoride raw material F. After melting the fluoride raw material F, the crucible C is slowly lowered from the hot zone HZ to a cold zone CZ, as shown in FIG. 1B. The cold zone CZ is at a temperature lower than the melting point of the fluoride raw material F. As the crucible C passes from the hot zone HZ to the cold zone CZ, the molten material MF goes through a zone having a thermal gradient designed to grow a good crystal (crystal growth thermal gradient) On passing through this zone, the temperature transition inside the molten material MF creates a crystal front CF. The crystal growth front CF propagates inside the crucible C, within the molten material MF, as long as the crucible C is caused to move downwardly.
The Gradient Freeze technique is illustrated in FIGS. 2A and 2B. In FIG. 2A, a crucible C containing a fluoride raw material F is disposed inside a vertical furnace 3. The vertical furnace 3 is provided with two heaters 4 for creating an axial crystal growth thermal gradient within the crucible C. A single heater that is capable of creating a crystal growth thermal gradient across the axial axis of the crucible C may also be used. The vertical furnace 3 is heated to a temperature sufficient to melt the fluoride raw material F. After melting the fluoride raw material F, the power applied to the heaters 4 is decreased in a manner that allows a desired axial crystal growth thermal gradient within the crucible C to be sustained. As shown in FIG. 2B, as the power applied to the heaters 4 is decreased, the molten fluoride material MF is directionally solidified into a solid fluoride material SF.
The Traveling Heater method is illustrated in FIGS. 3A and 3B. In FIG. 3A, a crucible C containing a fluoride raw material F is disposed inside a vertical furnace 5. The furnace 5 includes three heaters 6 for creating a desired axial thermal profile within the furnace. Alternatively, two heaters capable of creating a desired axial thermal profile within the furnace may be used. The crucible C is initially located in the upper section of the vertical furnace 5 and heated to a temperature less than the melting point of the fluoride raw material F. As shown in FIG. 3B, the crucible C is then moved into the middle zone of the furnace 5 that is at a temperature above the melting point of the fluoride raw material. At this position, a portion of the fluoride raw material is melted. As the crucible C moves relative to the furnace, the molten raw material MF re-solidifies into xe2x80x9cgoodxe2x80x9d material SF. The melted raw material MF moves continually inside the crucible C until all the material inside the crucible C has been re-solidified.
Typically, crystals are produced one at a time in either a net size or a large size that is machined to a desired shape. In either case, productivity is low and production cost is high. The number of crystals produced per furnace run, which require minimal machining, can be increased by loading one or more stacks of crucibles within the furnace, where each crucible contains a fluoride raw material. Multiple crystals can then be produced per furnace run using any of the techniques described above. However, using vertically-stacked crucibles does not automatically improve productivity. Productivity can still be hampered by dead times for loading the furnace, evacuating the furnace, heating the different thermal zones of the furnace, cooling the different thermal zones of the furnace, and unloading the furnace. Also, production costs can prove prohibitive if too many crystals are grown in a single furnace run. Up till now, selection of the number of crystals to produce per furnace run has been arbitrary.
In one aspect, the invention relates to a method for producing optical fluoride crystals which comprises loading a fluoride raw material into a vertical stack having at least 6 crystal growth chambers, heating the vertical stack to a temperature sufficient to maintain the fluoride raw material in a molten condition, applying a crystal growth thermal gradient to the vertical stack to form crystals within the molten fluoride raw material, and cooling the crystals.
In another aspect, the invention relates to a method for producing optical fluoride crystals which comprises loading a fluoride raw material into multiple vertical stacks, wherein at least one vertical stack has at least 6 crystal growth chambers. The method further includes heating the vertical stacks to a temperature sufficient to maintain the fluoride raw material in a molten condition, applying a crystal growth thermal gradient to the vertical stacks to form crystals within the molten fluoride raw material, and cooling the crystals.
In another aspect, the invention relates to a device for growing optical fluoride crystals which comprises a furnace having a capacity to hold a vertical stack having at least 6 crystal growth chambers and at least one heating element to maintain an appropriate treatment temperature inside the furnace.
Other features and advantages of the invention will be apparent from the following description and the appended claims.